You don't need rocks at all. Make sure you have a deep bed of hot coals and use hardwood. http://cowgirlscountry.blogspot.com/...derground.html
Don't let any air get into the pit.. check for smoke and throw more dirt on if it smoke is seeping out. Oxygen in the pit will cause flair ups.
No need for banana leaves or corn husks. Foil works fine also a layer of wet burlap.
It takes 3 or 4 hours to burn enough wood to get a deep bed of coals so either start very early in the morning OR (what I like to do) Burn the wood the night before, bury the pig and let it cook overnight.
Plan on at least 12 hours in the pit.
It never fails... the closer you get the pig to the pit, the smaller the pit looks and the bigger the pig gets.
Sometimes you just have to measure and dig some more
....and make sure you have enough room for at least 12" of hot coals, and about a foot of space around the pig.